a) Field of the Invention
The present invention relates to a combination lens system (a lens system which is composed of a plurality of lens elements and used as a lens), and also an image pickup lens system which is composed of a small number of lens elements and to be used with electronic image pickup apparatus such as TV cameras.
b) Description of the Prior Art
An optical system which is to be used under a white light source, for example an optical system for silver salt photographic cameras, microscopes, video cameras or endoscopes, must ordinarily be composed of a plurality of lens elements for favorably correcting aberrations such as chromatic aberration and a Petzval's sum. In addition to the requisite for correction of aberrations, it is generally demanded for reduction of a manufacturing cost, for example, to reduce a number of lens elements or simplify a structure of a lens barrel. As means for solving such a problem, it is conventionally known to use radial type gradient index lens elements. Since a radial type gradient.index lens element has a characteristic in that it is excellent for correcting chromatic aberration and a Petzval's sum in particular out of aberrations, it is effective to ruduce a number of lens elements which are used for composing a lens system to be used under a white light source. An optical system disclosed by Japanese Patent Kokai Publication No. Sho 60-218614, for example, is known as a conventional example of photographic lens system for silver salt photographic cameras which uses a radial type gradient index lens element. This conventional example is composed of two lens elements including a radial type gradient index lens element and equivalent to a Gaussian type optical system which conventionally reuqired six or seven lens elements. Further, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 5-107471 is known as a conventional example of objective lens system for endoscopes which uses radial type gradient index lens elements. This conventional example is composed of two to six lens elements and equivalent to a retrofocus type lens system which conventionally required lens elements in a number of six or so. However, each of these conventional examples adopts spherical surfaces for the radial type gradient index lens element and can hardly allow optical axes of surfaces with that of medium in practical working stages with high precision, thereby being not preferable from a viewpoint of obtaining a lens system which has high optical performance or can be manufactured at low costs.
In the recent years where video cameras using solid-state image pickup devices such as CCD's, TV telephones and door phones with cameras are prevailing, it is demanded that lens systems to be used in these instruments are configured compacter and manufactured at lower costs. Though three to six lens elements are generally required to obtain favorable optical performance of such a lens system which has a fixed focal point and is to be used in these instruments, it is desired to further reduce the number of lens elements.
As means for reducing a number of lens elements to be used for composing an optical system while maintaining optical performance desired for the optical system, it is conventionally known to use radial type gradient index lens elements in the optical system. For example, a lens system disclosed by Japanese Patent Kokai Publication No. Hei 6-175016 is known as a conventional example of lens system which is composed of a single radial type gradient index lens element. However, this conventional example has an extremely large F number of 9.8 to 13.5 or small aperture for balancing aberrations or enhancing imaging performance. Further, a lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-218614 is known as a conventional lens system which is composed of two lens elements. This conventional example has favorably corrected aberrations, but requires a high manufacturing cost due to a fact that it uses two radial type gradient index lens elements. In addition, the radial gradient index lens elements used in this lens system has spherical shapes, whereby an optical axis of the surfaces and an optical axis of medium can hardly be coincident with each other with high precision in a practical working stage, and the lens system is not preferable from viewpoints of high optical performance and low manufacturing cost.
Though it is conceivable to use a radial type gradient index lens element without working its surface into spherical shapes or in a condition where the lens element has planar surfaces on both sides, such a radial type gradient index lens element is not preferable since it under-corrects a Petzval's sum more remarkably as compared with a case where it has spherical surfaces.
Further, Japanese Patents Kokai Publication No. Sho 58-59420 and No. Hei 4-114112, for example, disclose conventional examples in each of which a radial type gradient index lens element having planar surfaces on both sides is cemented to a planar surface of a homogeneous lens element having a spherical surface on the other side for enlarging freedom for correction of aberrations. However, these conventional examples are pickup lens systems which are to be used under monochromatic light sources and these Japanese Patent Kokai Publications made no reference to correction of chromatic aberration. Japanese Patents Kokai Publication No. Hei 1-28514 and No. Hei 2-284107 which disclose similar conventional examples neither make reference to correction of chromatic aberration.
Furthermore, lens systems disclosed by European Patent Laid-Open No. 609093 and Japanese Patent Kokai Publication No. Sho 58-184113 are known as conventional examples of lens systems each of which is to be used under white light source, and is composed of a radial type gradient index lens element and a homogeneous lens element cemented to each other on planar surfaces. These are conventional examples using the radial type gradient index lens elements in relay lens systems for non-flexible endoscopes and usable only as lens systems such as the relay lens systems for non-flexible endoscopes having narrow field angles since the lens systems have large Petzval's sums and total lengths which are large relative to diameters of the lens systems.
Moreover, a conventional example disclosed by Japanese Patent Kokai Publication No. Sho 50-29238 is known as a lens system which is configured taking chromatic aberration into consideration. However, this conventional example uses radial gradient index lens elements which have refractive indices largely varying from optical axes to marginal portions and made of materials hardly manufactured in practice.